Estimation of the Gas in Place for Marcellus Shale Horizontal Wells by Rate Transient Analysis

Shale reservoirs are most commonly developed by horizontal wells coupled with multistage hydraulic fracturing to create a stimulated volume around the well. The rate transient analysis (RTA) has been used to estimate of the gas in place based on the production and flowing pressure data. RTA is particularly useful for the shale reservoirs where shut-in pressure data are rarely available. The application RTA to the shale reservoirs is however challenging because of the presence of the adsorbed gas, due to high organic content, and ultra-low permeability which requires a very long time for establishing the boundary-dominated flow (BDF). However, in a horizontal well with multiple hydraulic fracture stages, the interference between the fracture stages leads to an early BDF period. Therefore, the production data from this early BDF period can be utilized to determine the gas in place which can be attributed to the stimulated reservoir volume (SRV) which is economically more significant. The objective of this study was to investigate the applicability of the RTA for estimation of the gas in place associated with a Marcellus shale horizontal well with multiple hydraulic fracture stages. Furthermore, the impact of the adsorbed gas, shale compaction, and hydraulic fracture properties on the estimated gas in place were investigated. The available production data from several horizontal Marcellus shale wells in West Virginia were collected and analyzed to investigate the applicability of RTA for estimating the gas in place. The collected information from the horizontal wells were also utilized to develop a reservoir model for a Marcellus shale horizontal well with multiple hydraulic fracture stages. To accurately simulate the production performance, the adsorbed gas as wells as geomechanical factors, derived from the laboratory and published data, were incorporated in the model. The geomechanical factors account for the impairments in hydraulic fracture and shale (matrix and fissure) properties caused by the reservoir depletion. The model was then employed to generate a number of production profiles to investigate the impacts of the adsorbed gas, shale compaction, and hydraulic fracture properties. The results of the analysis provided reasonable estimates of the gas in place. Gas desorption was found to have insignificant impact on the estimation of the gas in place. Fracture half-length and shale compaction significantly impact the gas in place estimation. When the hydraulic fractures are closely spaced, the identification of the early BDF becomes difficult leading to uncertainty in the estimated of the gas in place. However, when the fracture stages are spaced widely, the early BDF can be identified more readily.